1. Field of the Invention
The present invention relates to a lighting unit and a luminaire to be used for street and/or road lighting. In particular, the present invention is directed to a lighting unit providing the possibility to adapt the light distribution in longitudinal direction of the road.
2. Related Technology
As it is customary with luminaires for street and/or road lighting in urban areas, known street lighting luminaires are mounted on a pole or on a wall front, with the plane of symmetry being directed perpendicular to the longitudinal direction of the street or road to be lit. The luminaire must then throw the generated light sideways from the plane of symmetry so as to evenly light parts of the street or road on the left and right of the luminaire. Further, a portion of the light is also irradiated along the plane of symmetry so as to also illuminate the whole width of the street or road in front of the luminaire.
When developing a luminaire for street lighting, different needs have to be addressed. At first, a fast and efficient installation of the luminaires is desirable. Further, a high utilization factor and efficiency is needed while at the same time the glare rating should be kept as low as possible. As in all lighting applications, a good visibility level should be obtained and the contribution to sky glow should be minimized.
From the prior art, different solutions for road lighting luminaires are known which can generally be divided into two main families.
Conventional road lighting luminaires are usually equipped with one lamp and a corresponding reflector system wherein the reflector system is designed to influence the light in such a way that a desired light distribution is obtained.
Further, road lighting luminaires are known which comprise a plurality of light emitting devices, in particular LEDs. In this case, the LEDs are arranged in a very specific pattern wherein each LED emits its light into a specific direction. In both cases, the known luminaires are regularly spaced on one side or both sides of the road.
The conventional luminaires mentioned above in most cases comprise a high-pressure sodium discharge lamp or a metal halide discharge lamp for which the emitted light of the burner is controlled by a reflector to obtain the desired street lighting. It is known to adapt the light distribution to different desired lighting schemes (i.e., a specific distance between the luminaires or a specific width of the read) by changing the position of the lamp with respect to the reflector. However, the range in which the light distribution can be adapted in this way is limited and also the utilization factor of such conventional luminaires is limited to a value of 0.45 to 0.5. In order to obtain a significant change in the light distribution characteristic of these luminaires, it would be necessary to change the reflector or use additional optical elements which influence the emitted light.
With respect to the second main family wherein a luminaire is based on a plurality of light emitting devices, different approaches have been proposed. According to a first solution, several light sources with different tilts and orientations are provided to obtain a combined light distribution suitable for road lighting. However, this approach tends to increase the complexity of the system and a limitation of glare is difficult to obtain. In another approach, all the light sources are arranged on the same planar surface and the lighting head of the luminaire is tilted by a relatively high angle (approximately 20° to 30°) in order to obtain an asymmetrical lateral light distribution which is required to illuminate also the opposite side of the road.